Slurry processes, particularly those occurring in bubble columns, are widely reported in scientific literature and, hence, are known to those skilled in the art. An example of such a slurry process is the production of hydrocarbons by means of the Fischer-Tropsch process.
Typically, a Fischer-Tropsch process is conducted by contacting a stream of synthesis gas (comprised mostly of hydrogen and carbon monoxide) with a liquid suspension of solid catalyst. The gas phase generally will have an H2/CO molar ratio of from 1:1 to 3:1. The dispersing liquid is primarily a mixture of linear paraffinic hydrocarbon reaction product. The gas is fed into the bottom of a “bubble column reactor” through a gas distributor which produces small gas bubbles that operate to suspend the catalyst particles in the liquid. As the synthesis gas rises through the column, it is converted mainly to hydrocarbon products that are liquids under the reaction temperature and pressure conditions. Those gaseous products that are formed rise to the top of the reactor from which they are removed.
Because it is necessary to maintain the slurry in the reactor at a constant level, liquid products are continuously or intermittently removed from the reactor. In doing so, however, it is important to separate dispersed catalyst particles from the liquid being removed to maintain a constant inventory of to catalyst in the reactor. Generally, the separation is conducted in a filtration zone located in the slurry bed. The filtration zone typically comprises cylindrical filtering media through which liquid product passes from the exterior to the interior where it is collected and removed from the reactor. In some reactor designs, liquid product is filtered in an external filtration zone and the separated catalyst is returned to the reactor.
One of the problems associated with filtration systems is the decrease in filter efficiency over time which necessitates remedial action such as backwashing the filter media, removing and cleaning the filter element or replacing it. The decrease in filter efficiency is due mainly to the presence in the liquid product of very small catalyst particles known as “fines,” which create an increasingly impervious solids cake over the filter requiring increasingly frequent backwashing, and which over time plug the filters. The presence of catalyst fines in the reactor is due to the attrition of the catalyst that occurs over time under the turbulent hydrodynamic conditions existing in the reactor vessel.
A number of ways have been proposed to remove fines, but these have not been satisfactory. For example, gravitational settlers have been proposed, but because the fine particles are so small, they do not settle out in a practical time period. Magnetic separation has also been proposed but has proven to be ineffective. Use of inclined plates, hydrocyclones and similar separation devices also are inefficient in separating fines from coarse particles particularly at high slurry solid loadings. Consequently, there is a need for increasing the effectiveness of separating catalyst fines from a slurry of bulk catalyst in a three-phase process containing gas and slurry.